1. Field of the Invention
This invention relates to a synchronous motor of the compact type and particularly to a reduction gear train incorporated therein.
2. Prior Art
One conventional synchronous motor 10 shown in FIGS. 1 and 2 comprises a first stator 11 of a cylindrical shape having a closed bottom, a second stator 12 of a circular shape having a peripheral flange 12a and snugly fitted in the first stator 11 against movement, a coil 13 wound around a bobbin 14 interposed between the second stator 12 and the bottom of the first stator 11, a rotor 15 in the form of a multi-pole permanent magnet mounted on a shaft 16 journalled in the bottom of the first stator 11 and a cover member 17 covering an open top of the first stator 11, and a reduction gear train 18 arranged between the cover member 17 and the second stator 12. The first stator 11 has stamped-out portions 11a directed toward the second stator 12 while the second stator 12 has stamped-out portions 12a directed toward the first stator 11. These stamped-out portions 11a and 12a serve as magnetic poles and are disposed in surrounding relation to the rotor 15. The rotor 15 has a plurality of poles of alternate north and south polarity and is rotated about the shaft 16 in synchronism with a rotating magentic field caused by the excited coil 13. The rotation of the rotor 15 is transmitted through the reduction gear train 18 to an output shaft 20.
The synchronous motor 10 has means for limiting the rotation of the rotor 15 in its reverse direction. More specifically, the rotor 15 has an elongated engaging portion 21 formed integrally on its upper surface, the engaging portion 21 extending diametrically of the rotor 15, as shown in FIG. 2. A retaining plate is rotatably mounted on a boss of a first gear member 22 of the reduction gear train 18, the retaining plate having an abutment lug 23a formed at one end thereof and directed toward the rotor 15. The retaining plate is disposed below a gear portion 22a of the first gear member 22 and is held in frictional engagement therewith so that the retaining plate is rotatable with the gear member 22. When the rotor 15 is rotated in its reverse direction at the initiation of the operation of the synchronous motor 10, the abutment lug 23a intrudes into a circle generated by the opposite end faces of the engaging portion 21 so that one of the opposite ends of the engaging portion 21 is brought into striking engagement with the abutment lug 23a. Upon striking of the one end of the engaging portion 21 against the abutment lug 23a, the engaging portion 21 rebounds from the lug 23a so that the rotor 15 is caused to rotate in its normal direction. When the rotor 15 is rotated in its normal direction, the lug 23a is moved out of the circle generated by the opposite end faces of the engaging portion 21 and is brought into contact with a stop member (not shown). Thus, the lug 23a is held against movement and will not interfere with the rotation of the rotor 15 in its normal direction. When the engaging portion 21 of the rotor 15 is brought into engagement with the abutment lug 23a of the retaining plate as a result of the rotation of the rotor 15 in its reverse direction, it is necessary that the north and south poles N and S of the rotor 15 should be located in predetermined angular positions relative to the magnetic poles 11a and 12a of the first and second stators 11 and 12 so as to properly effect the rotation of the rotor 15 in its normal direction. For example, in FIG. 2, when the engaging portion 21 of the rotor 15 is brought into contact with the abutment lug 23a of the retaining plate, the magnetic pole Sa is angularly displaced about the axis of the rotor 15 from a predetermined point of the magnetic pole 11a adjacent to the magnetic pole Sa by an angle .alpha.(11.25.degree.).
In the construction of the synchronous motor of this compact type, the positions of the rotor 15 and output shaft 20 are first determined, and then the gear members of the reduction train 18 are arranged sequentially in a direction from the output shaft 20 toward the rotor 15. With this arrangement, the position of the first gear member 22, which meshingly engages the rotor 15, must be varied when it is desired to change the number of revolutions of the output shaft 20. As a result, the position of contact of the engaging portion 21 with the abutment lug 23a is changed, and therefore the angular positions of the magnetic poles S and N of the rotor 15 relative to the magnetic poles 11a and 12a are also changed. In such a case, there are often occasions when the rotor 15 is not caused to rotate in its normal direction upon engagement of the engaging portion 21 with the abutment lug 23a. Therefore, conventionally, in order that the first gear member is always located in a predetermined position relative to the rotor 15 regardless of the number of revolutions of the output shaft 20, the positions of the magnetic poles N and S around the periphery of the rotor 15 are changed depending on the number of revolutions of the output shaft 20. However, with this method, various kinds of rotors having different magnetic-pole arrangements must be prepared depending on the number of revolutions of the output shaft 20. This increases the overall manufacturing cost of the synchronous motor since one common rotor can not be used. In addition, such rotors having different magnetic-pole arrangements can not be distinguished from one another from their appearance, and therefore a wrong rotor may be used in the manufacture of the synchronous motor, which will lead to a malfunction thereof.